|Nechols, James -|
Submitted to: International Symposium on Biological Control of Weeds
Publication Type: Proceedings
Publication Acceptance Date: November 10, 2011
Publication Date: January 1, 2013
Citation: Milbrath, L.R., Nechols, J.R. 2013. Plant-mediated interactions among herbivores: considerations for implementing weed biological control programs. International Symposium on Biological Control of Weeds. USDA Forest Service. FHET-2012-07:188. Technical Abstract: Complex trophic interactions are common, both in natural and managed ecosystems. One such interaction that has important implications for biological control of weeds involves plant responses to feeding by an herbivore which then impacts one or more other herbivores. Effects may be positive or negative, and mechanisms can be chemical or structural. Knowing if, and to what extent, these indirect plant-mediated interactions occur prior to importing new biological agents can assist with decisions about candidate selection, thus reducing economic and environmental costs, and increasing the overall success rate of weed biological control programs. We examined whether feeding by the musk thistle weevil Trichosirocalus horridus, which attacks the vegetative crown early in the plant’s development, alters musk thistle as a resource for the later-arriving weevil Rhinocyllus conicus, which infests flower heads. Minor infestations of musk thistle by T. horridus had no effect on R. conicus oviposition and subsequent production of new adults. In contrast, heavy infestations of T. horridus reduced 1) R. conicus-musk thistle synchrony, 2) acceptability of musk thistle to ovipositing R. conicus, 3) the quantity and 4) the quality of resource available to R. conicus larvae. As a result, the production of new R. conicus adults was reduced 63%. Thus, even spatially- and temporally-isolated herbivores can affect one another negatively and in multiple ways. Nevertheless, musk thistle seed reduction was still greater when both weevils were present. Hence, the outcome for biological control programs may not necessarily be adverse because of compensatory trade-offs concerning the relative impacts of the two herbivores on the weed. Recommendations for incorporating protocols to assess potential indirect plant-mediated impacts on weed biological control programs will be given.